1. Field of the Invention
The present invention relates to a belt conveyor and an image forming apparatus provided with this belt conveyor.
2. Description of the Related Art
A known image forming apparatus such as a color printer is provided with: a plurality of image bearing members (photoconductive drums) for bearing toner images of different colors; an intermediate transfer belt to which the toner images on the respective image bearing members are successively transferred in a superimposed manner by rotating a plurality of rollers; and a bending roller (pressure roller) for bending the intermediate transfer belt inwardly by coming into contact with the outer side of the intermediate transfer belt (see, for example, Japanese Unexamined Patent Publication No. 2004-45631). In this image forming apparatus, a color toner image formed on the outer surface of the intermediate transfer belt is transferred as a color image to a separately fed sheet. The sheet having the toner image transferred thereto is discharged to the outside after a fixing process is applied in a fixing unit.
In the image forming apparatus, the intermediate transfer belt is bent inwardly by the bending roller to make a space taken up by the intermediate transfer belt smaller and to make the apparatus smaller by arranging an other device (specifically, the fixing unit) in a space saved thereby.
When the outer circumferential surface of the bending roller comes into contact with the outer surface of the intermediate transfer belt, residual toner on the intermediate transfer belt adheres and deposits on the outer circumferential surface of this bending roller and this toner adheres to the intermediate transfer belt again in some cases. If the residual toner adhered again to the intermediate transfer belt is transferred to the sheet, a problem of reducing image quality is caused.
In the above image forming apparatus, a cleaning member for cleaning the outer circumferential surface of the bending roller is provided to solve such a problem. In this way, the outer circumferential surface of the bending roller is constantly kept clean. A drive motor for driving the bending roller is also provided to prevent the difficulty of the bending roller to rotate due to a resistance by the contact of the cleaning member with the outer circumferential surface of the bending roller.
By doing so, the bending roller reliably rotates according to the rotation of the intermediate transfer belt despite the resistance caused by providing the cleaning member. Further, the outer circumferential surface of the bending roller is constantly cleaned. In this way, the image forming apparatus prevents the occurrence of an image defect in a toner image transferred to a sheet.
However, since the intermediate transfer belt and the bending roller are respectively driven by different driving sources in the above image forming apparatus, it is very difficult to set the rotating speed of the intermediate transfer belt and the circumferential speed of the bending roller equal to each other.
Specifically, in the intermediate transfer belt, there are cases where the rotating speed of the intermediate transfer belt varies due to the rotation nonuniformity of the driving source, belt speed nonuniformity caused by an error in dimensional accuracy and the like. In the bending roller as well, there are cases where the rotating speed of the bending roller varies to change the circumferential speed of the bending roller due to the rotation nonuniformity of the driving source, the dimensional accuracy of the roller, the speed nonuniformity of the roller surface caused by twist accuracy and the like. Thus, it is impossible to constantly set the rotating speed of the intermediate transfer belt and the circumferential speed of the bending roller equal, and the outer surface of the intermediate transfer belt and the outer circumferential surface of the bending roller abrade against each other at different speeds in the above image forming apparatus.
If the outer surface of the intermediate transfer belt and the outer circumferential surface of the bending roller abrade against each other, there are cases where the intermediate transfer belt is scratched and a transfer potential is made nonuniform to cause a transfer failure. Further, there are cases where these surfaces are ground to produce fine particles and these fine particles adversely affect a toner image transferred to a sheet to reduce image quality.